This invention relates to liquid-gas contacting and has particular relationship to mist elimination in liquid-gas contacting. Liquid-gas contacting takes place in such operations as distillation or fractionation, gas scrubbing, evaporative cooling (water-cooling towers), trickle equipment for sewage aeration and the like. In such operations, the gas stream generally entrains the liquid as mist or drops as a result of the contact and it is often desirable that the liquid be removed from the gas. Mist eliminators serve the purpose of removing the liquid mist from the gas. In accordance with the teachings of the prior art, porous-pad demisters are provided for this purpose.
Mist eliminator pads may be made of woven or knitted mesh screen, expanded metal or of bonded non-woven construction, an example of which is described by Taylor. A typical widely-practiced method of making a pad mist eliminator is to knit or weave metal or plastic filament into a circular mesh sleeve or cylinder, which is then flattened, stacked and formed into a circular or rectangular pad to fit the appropriate containing vessel space. Another typical mist-eliminator pad material is described by Sinex, which discloses as a demister element an open-celled reticulated polyurethane foam manufactured by the Scott Paper Company, Chester, Pa. These materials and constructions and others similar to them yield a mist eliminator pad that is characterized macroscopically by uniformity of gas-flow resistance across the pad. In other words, microscopically the pores may be of random size and shape distribution, but macroscopically the gas flow is uniform. To minimize gas-flow resistance, the overall void space is generally very high, usually greater than 90%. Prior-art pads of this type have relatively high operating wet-gas-flow resistance as compared to the dry-gas-flow resistance (i.e., resistance to flow of gas containing no moisture). Such pads also manifest a tendency to fill with captured liquid mist, i.e., to flood. Flooding means that the mist-eliminator pad manifests a condition in which the liquid content of the pad rises rapidly, gas-pressure drop increases steeply, and ultimately liquid spray breaks through on the down-stream side of the pad.
It has been realized that the above-described functional disadvantages arise from the fact that both the upflowing gas and the down-draining trapped-out liquid mist must compete with each other for the same flow space within the interstices of the mist-eliminator pad. There are no effective preferred liquid-drainage channels or low-resistancce liquid flow paths, nor are there any low-resistance portions of the pad for preferential gas flow. The pad and its thickness present to the gas a macroscopically uniform flow transit resistance, indeed such uniformity is widely held to be desirable.
Where a demister pad is horizontally disposed so that the gas-flow is vertically upward or is inclined at an angle such that the gas-flow is prediminantly vertically upward, the liquid drains through the pad under the force of gravity. However, the drainage is hindered by the interacting interfacial tension forces near the up-stream side of the pad. The forces involved are the interacting molecular forces of the metal or other material of the pad, the gas, and the draining liquid. Another major factor hindering drainage of the demister pad is the momentum transfer and friction of the gas stream flowing wholly or partly counter to the direction of drainage.
As a result of these factors, the disentrained liquid collects as a continuous layer immediately above the upstream surface of the pad, or, in the case of an inclined pad, immediately above a large portion of this surface. The gas must then necessarily bubble as the discontinuous phase through the liquid layer, giving rise to high gas flow resistance relative to the dry pad and a ready tendency to flood, particularly at relatively low and moderate liquid loadings. Conventional entrainment elimination pads thus have limited liquid handling capacity and cannot operate at high liquid loadings. Generally such prior-art pads are limited to loadings less than approximately 0.5 GPM/sq. ft. of surface area. For this reason, it is customary to place the mist eliminator pad as far as possible from the source of the liquid entrainment, i.e., from the region from which the liquid-containing gas emerges, to permit gravity attenuation of the liquid carryover. This is not an economically satisfactory solution because the entrainment decay distance required is bought at the expense of additional shell or equipment volume to achieve this distance.
Sexton, Otto and Eckert disclose attempts to resolve the flow and pressure-drop limitations of the prior-art pad mist eliminators. Both Sexton and Otto employ mist eliminator structures disposed at a large angle to the horizontal to facilitate liquid-drainage along the inclined pad into either external drain channels, in the case of Sexton, or onto the interior circumference of the containing vessel wall, in the case of Otto. Eckert discloses the use of an undulating foraminous plate as a mist-eliminator support and liquid redistributor. It has been realized in considering the effectiveness of Eckert that the up-flowing gas and its accompanying mist loading must necessarily flow through the foramina in Eckert's undulating plate support, and that this plate would manifest the undesirable liquid-gas counterflow properties of the demister pad. The planar-surface, essentially two-dimensional, foraminous plate subjects the liquid-film-wetted surface in its holes to direct gas frictional drag. Because of the gas drag and impact on the unprotected liquid surfaces, the draining liquid is subject to retardation and hindered flow, and the foraminous support plate is itself subject to flooding. Further, interposition of any foraminous plate in a two-phase liquid-gas flow system causes an incremental gas-flow resistance over that of the pad alone, and added resistance to gas flow is economically undesirable.
It is an object of this invention to overcome the disadvantages and the deficiencies of the prior art and to provide a method of mist elimination and a mist eliminator which shall have a substantially higher liquid-handling capacity and shall be capable of operating at substantially higher liquid mist loadings than prior-art mist eliminators, without excessive pressure loss or flooding. It is also an object of this invention to provide such a mist eliminator in whose operation the liquid shall be effectively drained therefrom, while the liquid is protected from the frictional forces of the counterflowing gas stream, and the drained liquid shall emerge in large enough drops to resist reentrainment by the gas.